Cone-beam computed tomography (CBCT) is an important online imaging modality for image-guided radiotherapy (IGRT) as well as other forms of image guided interventions. However, current CBCT image quality is inferior to that of the diagnostic fan beam CT. The image quality problem is inherent in the design of the current CBCT system, which is composed of a point x-ray source and a two-dimensional flat panel imager (FPI). Excessive x-ray scatters, the use of low performance FPI, and reconstruction degradation at large cone angles are three major sources of the degradation of image quality. The inferior image quality of CBCT limits the use of this modality for important new IGRT treatment techniques. We have designed a novel quasi-CBCT scanning system comprising a linear scan x-ray source and a linear discrete x-ray detector array. This imaging system will overcome the defects inherent in FPI-based CBCT and thereby produce online images with diagnostic quality. The linear x-ray tube and detector array are aligned perpendicular to and within the rotation plane, respectively. The x-ray beams are narrowly collimated into fan beams and scan in the z-direction electronically. This system will produce diagnostic quality online images for IGRT due to its scatter rejection mechanism and high-performance discrete x-ray detectors. Besides improved image quality, this system also has larger clearance due to its slim structure. Diagnostic online images will facilitate new IGRT techniques with better tumor killing and normal tissue sparing. This innovative design will also have huge impact on other forms of image guided intervention as well as diagnostic imaging. A quasi-CBCT bench-top system will be built in this research. The feasibility and improvement of image quality of the quasi-CBCT system will be studied. Techniques and parameters for clinical implementation will be obtained. NARRATIVE In this project, we will develop a novel quasi cone beam computed tomography (CBCT) system for image guided radiotherapy (IGRT) as well as other forms of image guided interventions. This system will solve the inherent problem of current flat-panel based CBCT and will produce online images with diagnostic quality. Clinical implementation of this innovative online imaging system will facilitate new IGRT techniques that are impossible with the inferior image quality of current CBCT. Once developed, this mobile and diagnostic CT imaging system will have huge impacts on many other forms of medical practice.